1. Field of the Invention
The present invention relates to a control circuit for a power converter, and more specifically relates to a switching control circuit for a switching mode power converter.
2. Description of Related Art
Power converters have been widely used to provide regulated output voltages. For safety purposes, an off-line power converter must provide galvanic isolation between its primary side and secondary side. An optical-coupler and a secondary-side regulator are needed to regulate the output voltage of the off-line power converter. In order to reduce the device counts and to exclude a feedback circuit from a secondary-side of the power converter, a control technology at a primary-side has been disclosed in U.S. Pat. No. 4,302,803 “Rectifier-Converter Power Supply with Multi-Channel Flyback Inverter”, by Randolph D. W. Shelly. In recent technology, U.S. Pat. No. 6,721,192 “PWM controller regulating output voltage and output current in primary side”, by Yang, et al., U.S. Pat. No. 6,836,415 “Primary-side regulated pulse width modulation controller with improved load regulation”, by Yang, et al., U.S. Pat. No. 6,853,563 “Primary-side controlled flyback power converter, by Yang, et al., and U.S. Pat. No. 6,862,194 “Flyback power converter having a constant voltage and a constant current output under primary-side PWM control”, by Yang, et al, have been introduced. However, for measuring a signal from the transformer, the foregoing technologies cannot yield accurate result.
The objective of the present invention is to provide a precise sampling circuit for a switching control circuit of a power converter for measuring a reflected voltage signal and a discharge time of a transformer. By using the sampled signal from the transformer, the output of the power converter can be regulated without the optical-coupler and the secondary-side regulator. In addition, the discharge time of the transformer is able to be used for quasi-resonant power converters to synchronize with the valley voltage and to achieve soft switching effect.